Please can I get help with Battery usage by apps ?

After getting the AU in my mobile,I find that limiting battery usage by apps gives 3 options -Always allowed in background,Managed by Windows & Never allowed in background.What does 'managed by Windows' imply ? Which apps should I set to 'managed by Windows' and which to 'always allowed '?
Thanks !

Thanks !
But do apps work in the background (those I need to do so ) if I don't allow to 'always work in the background' ?
I mean that why are the two options given separately ? There must be some difference.

Thanks !
But do apps work in the background (those I need to do so ) if I don't allow to 'always work in the background' ?
I mean that why are the two options given separately ? There must be some difference.

Only few, like Messages, Messenger, Phone, but with limited functions, like phone sometimes shows numbers instead contacts or unable to block numbers in black list.
Skype in background drains like a truck, standing by new beta uwp. For others, consider everyhing you don0t use everyday. That's the same like a pc, the difference is on desktop that's called apps allowed startin on every system start.